In the manufacturing industry, particularly the wire drawing industry, wires (both round and of angular shapes) and small tubes are cold reduced in size by drawing or rolling long strands of the product. Because not all of the cold reduction needed to obtain the final shape can always be obtained without intermediate annealing, the wires are typically continuously annealed by passing them through a long tube or pipe within a furnace which is usually heated by electrical resistance or gas firing and operating at a sufficiently high temperature to cause softening of the cold worked wire. This technique has a number of advantages (along with some disadvantages) over batch annealing but its primary asset is the uniform heating of the product without the harmful effects of extended exposure of the wire to elevated temperatures. Temperatures within the tube may reach as high as 2300.degree. F., but more common are temperatures between 500.degree. F. and 2200.degree. F. depending upon the alloy or product being annealed.
To further protect the product, the atmosphere inside the tube is typically a slowly flowing protective or reducing atmosphere such as argon, hydrogen, cracked ammonia or the like. In some installations, similar tube furnace arrangements may be used with a carburizing or nitriding atmosphere flowing through the tubes in order to chemically treat the wire product as it passes through the furnace. The atmosphere surrounding the outside of the tubes is usually heated air often containing combustion gases. Thus, the tubes or pipes are of necessity made of an expensive alloy which is designed to withstand the high temperatures for extended periods of time. Typical oxidation resistant materials for annealing tubes include INCONEL alloys 600 and 601, INCOLOY 800, HAYNES 214 alloy and the austenitic stainless steels. Extending the service life of such tubes is a prime concern of the industry.
Unfortunately, the theoretical service life of these tubes is often reduced substantially by a combination of clogging, corrosion and/or cracking (and eventual breakage) near the entrance to the hot zone of the tube. The clogging of the internal passageway is most often attibuted to residual lubricant on the wire when it enters the annealing tube. Typically, lubricants are used during the cold working operation to prevent damage to both dies and the wire or product being manufactured. One of the prerequisites of such a lubricant is that it adheres tenaciously to the wire and that it be relatively stable to moderate temperatures, perhaps 150.degree. F. This requirement results from the heat created by friction during the drawing operation. Because of the higher furnace temperature, the lubricant melts, drips from the wire, collects on the inside of the annealing tube, and builds up as a hard, cementaious deposit when volatile portions of the lubricant are swept out of the tube by the protective gases. Sometimes the deposit is formed from the combined effect of lubricant and small flakes or chips from the wire. This clogging process usually occurs over a relatively short region close to the entrance end of the tube while most of the tube is less obstructed. In some instances, probably because of variations in temperature, wire speed, or drawing compounds, the clogging occurs closer to the exit end of the tube. These clogged tubes are routinely removed and scrapped even though much of the tube would have significant service life remaining.
Another mode of premature failure of strand annealing tubes is cracking and fracture a short distance inside the entrance end of the furnace leaving a too short (but otherwise serviceable) tube which must be discarded. Three factors are now hypothesized to contribute to these failures. These are (1) accelerated corrosion caused by the dripping and partial volatilization of drawing compounds attacking the metal, (2) thermal shock to the metal tubes caused by relatively cold (maybe 250.degree. F.) drawing compound or steam from damp wire contacting the hot (up to 2300.degree. F.) tube wall, and (3) mechanical abuse. The most common source of mechanical abuse is the vibrations caused by the product to be annealed entering the annealing tube at an angle to the length of the tube. As the wire rubs on the tube at an angle, the force can be divided into at least two components. One which tends to cause the tube to move from its longitudinal axis thereby causing the vibration, and the second which is more or less parallel to the longitudinal axis of the tube, which tends to cause the tube to deviate from linearity in much the same manner as a straight string "snakes" when one end is pushed.
The obvious solutions to the above stated problems would be to clean and dry the product before it enters the annealing furnace to avoid the clogging of the tubes and/or avoid the mechanical abuse mentioned above. To date, cleaning the entering wire is often attempted, but is frequently inadequate. The vibrations caused by the non-alignment of the wire with the centerline of the tube can be eliminated, or at least reduced, by using a guiding mechanism to align the wire with the tube centerline, but for reasons peculiar to individual operations, this is not always feasible.
For many reasons, such as capital equipment costs, space limitations, or characteristics of the drawing compound or wire, the apparent solutions to the problems can not always be utilized. The result is untimely and costly work stoppages, lost production time, increased labor costs for maintenance, and new purchases of costly replacement tubes.